The invention relates to voltage regulators having low dropout voltage. In particular, linear complementary metal oxide semiconductor (CMOS) technology is used wherein diffused metal oxide semiconductor (DMOS) devices are included. A large area self-isolated DMOS transistor can pass a substantial current that is linearly related to a control potential. This characteristic is disclosed and claimed in my copending patent application Ser. No. 189,442, filed May 2, 1988. This application is titled VOLTAGE MULTIPLIER COMPATIBLE WITH A SELF-ALIGNED C/DMOS PROCESS and is assigned to the assignee of the present invention. Its teaching is incorporated herein by reference.
Basically, I have found that a large area DMOS transistor operated from a relatively low voltage supply can couple a potential that is very close to that of the supply. This means that when the DMOS transistor is employed as a pass transistor in a voltage regulator, the dropout voltage can be very low. To do this the gate of the DMOS transistor must be operated at a potential that is substantially in excess of the supply potential. This is called overdrive. In the above-referenced patent application a DMOS transistor coupled to a 5-volt V.sub.DD supply will ordinarily be overdriven at a gate potential of 10 to 15 volts. Such an overdrive can be obtained from a voltage multiplier rectifier that is operated from a system clock pulse which ordinarily has a peak-to-peak value substantially equal to the V.sub.DD supply voltage. Thus, for a 5-volt system a voltage tripler will produce a bias level of almost 15 volts.